Working with Relationship Mappings

Persistent objects use relationship mappings to store references to instances of other persistent classes. The appropriate mapping class is chosen primarily by the cardinality of the relationship.

Specifying Private or Independent Relationships

In OracleAS TopLink, object relationships can be either private or independent.

• In a private relationship, the target object is a private component of the source object. The target object cannot exist without the source and is accessible only through the source object. Destroying the source object will also destroy the target object.

• In an independent relationship, the source and target are public objects that exist independently. Destroying one object does not necessarily imply the destruction of the other.

Aggregate object mappings are private by default, because the target object shares the same row as the source object. One-to-one, one-to-many, and many-to-many mappings can be independent or private, depending upon the application. Normally, many-to-many mappings are independent by definition; however, because a many-to-many mapping can be used to implement a logical one-to-many without requiring a back reference in the target to the source, OracleAS TopLink allows many-to-many mappings to be private as well as independent.

Tip: OracleAS TopLink automatically supports private relationships. Whenever an object is written to the database, any private objects it owns are also written to the database. When an object is removed from the database, any private objects it because are also removed. Be aware of this when creating new systems, since it may affect both the behavior and the performance of your application.